1. Field of the Invention
The present invention relates to a still/slow circuit for use in a two-head type video cassette tape recorder (hereinafter "VCR") and more particularly, to a circuit for use in a VCR of the two-head PAL system which performs smoothly the still and slow playback operations with existing two heads.
2. Description of the Prior Art
Conventionally, several types of still and slow circuits provided the function of the still and slow playback in a VCR having four heads or heads for still function are well known in the art.
FIG. 1 is a block diagram show in one of such conventional apparatuses for still/slow function, which comprises an amplifier 1 for amplifying playback control pulses CP, a rising edge detector 2 for detecting rising edges out of outputs from the amplifier 1, first and second monostable multivibrators 3 and 4 for generating signals of a predetermined width in response to the detected signals of the rising edge detector 2, a third monostable multivibrator 5 for generating signals of a predetermined width when the output of the second monostable multivibrator 4 is finished, a first flip-flop 6 for outputting high potential pulses which are drive initiation voltage during signals of a predetermined width is outputted from the monostable multivibrator 3, a second flip-flop 7 for outputting high potential pulses which are normal drive voltages during signals of a predetermined width is outputted from the second monostable multivibrator 4, a third flip-flop 8 for outputting low potential pulses which are stop voltages during signals of a predetermined width is outputted from the third monostable multivibrator 5, and a fourth monostable multivibrator 9 for making the first, second, and third flip-flops 6, 7, and 8 enabled by a still indication signal ST or a slow indication signal SL.
Such conventional circuit having the configuration as above operates as follows:
A normal playback capstan control circuit is stopped its operation at the time of still or slow operation, while the fourth monostable multivibrator 9 makes the first, second, and third flip-flops 6, 7, and 8 enabled by the still indication signal ST or the slow indication signal SL.
As a result, the playback control pulses as shown in FIG. 2A are amplified at the amplifier 1 and applied to the rising edge detector 2 at which rising edges are detected. By these detected signals, signals of a predetermined width as shown in FIGS. 2C and 2E are outputted from the first and second monostable multivibrators 3 and 4. At this time, the widths of the output signals of the second monostable multivibrator 4 can be varied properly by varying a variable resistor ST for slow tracking.
The signals outputted from the second monostable multivibrator 4 causes the third monostable multivibrator 5 to output the signals of a predetermined width as shown in FIG. 2G.
At the first, second, and third flip-flops 6, 7, and 8, signals of the waveforms are outputted as shown in FIGS. 2D, 2F, and 2H respectively, during the signals of a predetermined width are outputted from the first, second and third monostable multivibrators 3, 4, and 5. The output signals of the first, second, and third flip-flops 6, 7, and 8 are so mixed as shown in FIG. 2B, as to be applied as a capstan motor control signal, so that the still or slow operation can be carried out.
That is, the high level region represents a drive initiation region, the medium level region represents a normal drive region, and the low level region represents a stop period in the waveforms as shown in FIG. 2B.
However, such conventional apparatus can only be applied to a VCR provided with heads for still operation or a four-head of double azimuth and it requires expensive parts and complicate structure. Specifically, in a two-head type, two heads trace in turn two tracks to perform a normal playback, and in case when the normal playback operation is stopped, the tracing slant angles thereof become different from each other, which causes the two heads not to be precisely accorded to the two tracks. That is, when the two heads are precisely accorded to the two tracks any noise is not produced, while they are not precisely accorded to the two tracks a noise is produced. There is not provided any means to control such a noise it is not possible to be utilized in the VCR of two-head type. And the existing two-head type VCR is not possible to perform the slow and still playback operations.